Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define those millions of magnetic tunnel junction (MTJ) cells in each MRAM device and make them non-interacting to each other, precise patterning steps including photolithography and reactive ion etching (RIE), ion beam etching (IBE) or their combination are usually involved. During photolithography, patterns are transferred from a photomask to a light-sensitive photoresist and later transferred to MTJ stacks by RIE, IBE or their combination, forming separate and non-interacting MTJ devices. Therefore the MTJ cell size is generally determined by the photoresist pattern size. To fabricate future sub-nano node products with ultra-small device size, various complex and expensive photolithography systems such as immersion deep ultraviolet (DUV) and extreme ultraviolet (EUV) are needed. A simple low cost alternative solution would be preferred.
Several patents teach using more than one hard mask. U.S. Patent Application 2010/0327248 (Khoueir et al) and U.S. Pat. No. 9,543,502 (Zou et al) and U.S. Pat. No. 9,722,174 (Nagel et al). All of these references are different from the present disclosure.